These are LaTeX
delcarations (with the exception of
\emph),
i.e., they do not act on an argument, but change the way LaTeX prints the
following text. The scope of a declaration is ended by a right
brace (}) or an \end{} command whose matching
left brace ({) or \begin{} precedes the declaration.

Thus, for a short piece of italics or bold-face text one would use

This is {\it italics} and this is {\bf in boldface}.

which would be set as "This is italics and this is
in boldface.

In LaTeX2e the
\emph command is considered more
suitable for short stretches of emphasized text. A typical use would be